The background description includes information that may be useful in understanding the present inventive subject matter. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed inventive subject matter, or that any publication specifically or implicitly referenced is prior art.
Friction stir welding (“FSW”) is a solid-state welding process in which a rotating tool heats and intermixes two workpieces at a seam (e.g., a junction, joint, or boundary between the workpieces). More specifically, the rotating tool has a pin that is pressed into the seam as the tool rotates, producing frictional heat between the tool and the workpieces. Enough heat is generated such that regions of the workpieces plasticize. A shoulder of the FSW tool assists in causing the plasticized regions to intermix, thus joining (i.e., FSW) the workpieces at the seam. The rotating tool travels along the entire length of the seam to form a weld joint line between the two workpieces.
FSW provides numerous advantages over other welding processes, in part, due to the fact that FSW occurs at much lower temperatures and without a filler material. Some of the advantages of FSW include: better mechanical properties at the weld; less porosity, shrinkage, and distortion; little or no toxic fume emissions; no consumable filler material; and ease of automation. Since its conception in 1991, FSW has been heavily researched and successfully applied to numerous industries in a wide variety of applications.
The ability to produce high quality and high strength welds has made FSW an attractive process for joining large-diameter pipe sections made of high strength, such as those used in transporting petroleum. However, in order to achieve a high quality weld, many different process parameters must be monitored and controlled (e.g., travel speed rate, rotational speed rate, alignment, pressure, temperature, etc).
Numerous references describe FSW systems that have sensors for monitoring and controlling FSW process parameters in order to achieve high quality welds. U.S. Pat. No. 5,893,507 and International Patent Application Publication No. WO 00/02704, for example, describe FSW systems that monitor and/or control pressure at the FSW tool tip using pressure sensors. U.S. Pat. No. 6,050,475 describes a method of maintaining a constant pressure at the FSW tool tip by using the current of a drive motor to estimate actual pressure. U.S. Pat. No. 6,299,050 describes a FSW system that monitors and controls the distance between the FSW tool and the workpieces using a sensor just ahead of the moving FSW tool. International Patent Application Publication No. WO 98/13167 describes a FSW system that uses an ultrasonic device to measure variations in the thickness of the workpieces in order to control the distance between the FSW tool and the workpieces.
Other examples of FSW processes that monitor and control process parameters using sensors are found in European Patent Application EP2094428, U.S. Patent Application No. 2012/0261457, and International Patent Application Publication No. WO 2012/133411.
These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Even when FSW process parameters are carefully monitored and controlled, a certain number of defects will inescapably be present in a weld. Examples of common defects include volumetric defects (e.g., gaps or voids), root flaws or weld line defects (e.g., a portion of the seam fails to bond along the weld line), joint line remnants (i.e., when remnants of undesirable joint line materials, such as surface oxidation, mix into the weld), and excessive flash (i.e., when workpieces are overly heated and the FSW tool ejects material from the weld region). Weld deflects reduce weld quality and can even cause the weld to fail (e.g., break, crack, leak).
There are a variety of destructive and nondestructive methods for detecting such defects, including both “offline” (e.g., after the welding is completed) and “online” (e.g., while the weld is still in process or the workpieces are still associated with the FSW device) methods. Online, nondestructive methods are often preferable, as they have the potential to allow repair of the defect while the workpieces are still in place and the equipment is in operation. Online methods are especially preferred when the FSW equipment is large, heavy, and difficult to transport.
Visual inspection is one example of an online nondestructive method for detecting defects. Visual inspect can be an effective, although labor intensive, method of identifying surface flaws. Visual inspection can be automated through the use CCD cameras and appropriate image processing hardware and software.
Radiographic testing is another example of an online nondestructive method for detecting defects in welds and can even be used to identify “hidden” defects (i.e., defects not readily observable with the natural eye). Radiographic testing operates by exposing the weld to a radiation source (e.g., emission from gamma radiation sources such as 192Ir, 60Co, and 137Cs) and measuring the amount of radiation that penetrates the workpiece. Radiographic testing provides information on the thickness and composition of the weld.
Another method for online nondestructive testing is ultrasonic characterization. This method provides information on the density distribution of materials and is well suited for detecting voids and root flaws. U.S. Patent Application Publication No. 2003/0057258, for example, discloses a FSW machine that includes an ultrasonic probe that trails the path of the FSW tool to identify and mark defects. U.S. Patent Application Publication No. 2009/0140026A1 and Japanese Patent Application Publication No. JP2004317475 also disclose the use of ultrasonic probes to characterize defects in FSW welds.
Yet another example of an online nondestructive testing method is eddy current detection, which yields information regarding the density and composition of material within the weld. Unfortunately, eddy current detection does not penetrate workpieces well and thus is limited to relatively thin workpieces. Examples of FSW systems that include eddy current detection of defects are found in U.S. Patent Application Nos. 2010/0117636A1, 2004/239317, and U.S. Pat. No. 6,168,066.
Other methods of detecting defects in welds involve inferring the presence of a defect based on correlative data. For example, “Monitoring and Control in Friction Stir Welding” by Paul A. Fleming (dissertation submitted to the Faculty of Graduate School of Vanderbilt University, May, 2009) describes utilizing acoustic and electromagnetic emissions generated by the FSW tool during the welding process to infer the presence of defects in a weld. Similarly, attempts have been made to train neural networks to associate force and torque data gathered during FSW with the presence of weld defects. While these attempts have shown some success, those of ordinary skill in the art have failed to produce adequate quantitative information about the presence and severity of specific defects to provide an online certification method.
As used herein, “online certification” as it applies to FSW, means to provide a certification of a FSW weldment while the FSW device is still associated with the workpieces that are being joined. As used herein, “associated” means that the FSW device is still attached to, in physical contact with, or at least within close proximity to (e.g., in the same room, building, or worksite), the workpieces. The terms “dissociation,” dissociated,” “disassociate” and the like are used herein to refer to a FSW device that is no longer associated with the workpieces and/or weld site (e.g., the FSW device has been detached from and/or removed from the worksite).
While various methods of detecting weld defects and flaws are known, these methods fail to provide an online nondestructive method for automatically or semi-automatically certifying that a weld complies with the standards of standard setting organization. Examples of standard setting organizations include the International Organization for Standardization (ISO), the American Welding Society (AWS), and the American Society of Mechanical Engineers (ASME). Since the certification of weld quality is critical in certain applications, it would be advantageous to determine whether a weld passes certification while workpieces and FSW devices are still associated with one another, thereby facilitating efficient repair of the flawed piece.
Thus, there remains a need for a system and method that provides online certification of friction stir welds.